1. Field of the Invention
This invention relates generally to combiners/decombiners, both of the selective wavelength type such as optical multiplexers/demultiplexers, for example, arrayed waveguide gratings (AWGs) or elliptical supergratings (ESGs) and of the non-selective wavelength type such as free space region couplers, star couplers or multi-mode interference (MMI) couplers. More particularly, this invention relates to the deployment of “tilted” optical combiners/decombiners of both the wavelength selective and the non-selective type.
2. General Definitions
Reference is made in this disclosure to a “optical combiner/decombiner” when only the application of the optical combiner itself is generally discussed. However, as it will be evident to those skilled in the art, the principals of such optical combiners shown in this disclosure can be equally be utilized as optical decombiners such as illustrated in an optical receiver photonic integrated circuit (RxPIC) disclosed in U.S. patent application, Ser. No. 10/267,304, filed Oct. 8, 2002 and published on Feb. 19, 2004 as Pub. No. 2004/0033004, which application is incorporated herein by its reference.
Reference in this description to signal channels is nominally either a modulated semiconductor laser source or a semiconductor laser source and its associated modulator, each referred to as a “modulated source” and providing a modulated signal output. In this context, such a signal channel is also an optical waveguide with one or more accompanying integrated elements, such as, but not limited to, a laser source, a modulator, a photodetector or a semiconductor optical amplifier (SOA), a variable optical attenuator (VOA) or a combination VOA/SOA. Thus, a train of elements formed in each signal channel comprising a modulated source and other accompanying integrated elements for a signal channel array comprising two or more such channels integrated in a photonic integrated circuit (PIC).
“Laser emission wavelength” means emission output wavelength of a laser source in a particular signal channel formed in a photonic integrated circuit (PIC) such as an optical transmitter photonic integrated circuit or TxPIC.
“Active region wavelength” means the wavelength of the photoluminescence (PL) peak or the gain peak of an active region wavelength spectrum, for example, of laser source and/or modulator formed in a photonic integrated circuit (PIC) signal channel where the channel laser source and modulator share the same active region and active region bandgap and/or share the same active region and active region bandgap with adjacent, integrated signal channels. For purposes of brevity in this invention, PL peak and gain peak are used interchangeably although they are slightly different as is known in the art.
The terms, “laser(s)” and “laser source(s)”, are used interchangeably in this disclosure.
An “active region” as employed in the description of this disclosure means the region in a semiconductor device where carrier recombination and/or light absorption occurs which may be comprised of a single semiconductor active layer or multiple semiconductor layers with any associated optical confinement layers, as is well known to those skilled in the art. An active layer, for example, may be a single high refractive index active layer or may be multiple active layers such as in the case of multiple quantum well layers and barrier layers which are, together, commonly referred to as an active region.
Also, examples of the inventive optical combiners/decombiners illustrated in this disclosure are referred to as “tilted” in the sense that the inputs or the outputs of the combiner free space region or regions are spaced from or offset relative to a longitudinal centerline of the free space region, but still remain within a zero order band of the combiner free space region, rather than being positioned on and about that centerline of a zero order band of an input free space region of the combiner, which is the common practice in such devices for optimized combiner passband results. In terms of the foregoing description, “offset” of the inputs or outputs, as the case may be, means that they are spatially to one side of the longitudinal centerline of the free space region, i.e., they are spatially offset.